If you compare it to the original this is much easier, maybe because of additional squares, or maybe because I can move in more directions. Either way the game did not become challenging for me (played until I had 2048 as my largest tile), I frankly got bored before it got difficult.

Maybe you could solve this by increasing the tempo of new tiles. E. g. throw in an 8, or 16 every once in a while. Or maybe add two tiles per movement sometimes.

Also, and this is more of a curiosity, I use the Dvorak keyboard layout and would have been nice if I could have used the same keys as Qwerty, even though I have different mappings. Don't know if it can be done and you really should not spend any time on it :)